IPV6 packet

An IPV6 packet is generally composed of the following parts:

·        IPV6 header

·        IPV6 extension header

·        Upper-layer protocol data unit

What is the difference about these parts???

• IPv6 Header - The IPv6 header is always present and is a fixed size of 40 bytes.
• Extension Headers - Zero or more extension headers can be present and are of varying lengths. If extension headers are present, a Next Header field in the IPv6 header indicates the first extension header. Within each extension header is another Next Header field, indicating the next extension header. The last extension header indicates the header for the upper-layer protocol—such as Transmission Control Protocol (TCP), User Datagram Protocol (UDP), or Internet Control Message Protocol for version 6 (ICMPv6)—contained within the upper-layer protocol data unit.

  The IPv6 header and extension headers replace the existing IPv4 header and its options. The new extension header format allows IPv6 to be enhanced to support future needs and capabilities. Unlike options in the IPv4 header, IPv6 extension headers have no maximum size and can expand to accommodate all the extension data needed for IPv6 communication.
• Upper-Layer Protocol Data Unit - The upper-layer protocol data unit (PDU) typically consists of an upper-layer protocol header and its payload (for example, an ICMPv6 message, a TCP segment, or a UDP message).

  The IPv6 packet payload is the combination of the IPv6 extension headers and the upper-layer PDU. Normally, it can be up to 65,535 bytes long. IPv6 packets with payloads larger than 65,535 bytes in length, known as jumbograms, can also be sent.

  
  

  
  
  
  

IPv6 Basic Header
An IPv6 basic header is fixed as 40 bytes long and has eight fields. Each IPv6 packet must have an IPv6 basic header that provides basic packet forwarding information, and which all devices parse on the forwarding path.

IPv6 Extension Header
An IPv4 packet header has an optional field (Options), which includes security, timestamp, and record route options. The variable length of the Options field makes the IPv4 packet header length range from 20 bytes to 60 bytes. When devices forward IPv4 packets with the Options field, many resources need to be used. Therefore, these IPv4 packets are rarely used in practice.

Thanks for your information.